The present invention relates to a method of receiving a signal in a mobile station in an art of spread spectrum communications, e.g., a method of receiving a signal utilized in a mobile communication system based on the CDMA (Code Division Multiple Access), and more particularly to a method of receiving a signal through RAKE reception using a plurality of demodulators and a radio communication terminal device for carrying out such a method.
Heretofore, in order for a number of mobile stations to share a single base station for communications, various encoding schemes including frequency division, time-division multiplex, and code-division multiple access are used to avoid interference between communication channels of the mobile stations. These encoding schemes have different features of their own, and are selected depending on the purpose of a communication system in which the encoding process is incorporated.
For example, the code division multiple access (hereinafter referred to as CDMA) spreads modulated waves having the same carrier frequency into a frequency band wider than the original frequency band using certain codes, e.g., PN (Pseudo random Noise sequence) codes assigned to respective channels (hereinafter the above process is referred to as spread-spectrum), multiplexes the spread-spectrum modulated waves, and transmits the multiplexed spread-spectrum modulated waves. A received spread-spectrum signal and a PN code given via a channel to be demodulated are synchronized to each other to identify the desired channel.
Specifically, a transmitting station assigns different PN codes to respective channels. The PN codes are pseudo random noise sequence codes. The transmitting station multiplies modulated waves to be transmitted via the respective channels by the different PN codes to spread the modulated waves over a spectrum of frequencies. The modulated waves have been modulated before they are spread. The spread-spectrum modulated waves are then multiplexed and transmitted.
A receiving station despreads the received signals from the transmitting station by synchronizing and multiplying the received signals by the same PN code assigned to the channel to be demodulated. In this manner, only the modulated wave transmitted via the desired channel is demodulated.
According to the CDMA, the transmitting and receiving station can communicate with each other for each call by employing the same code. Since modulated waves are spread using different PN codes assigned to respective channels according to the CDMA, the receiving station can only demodulate a spread-spectrum signal transmitted via the channel to be demodulated. Furthermore, the CDMA is highly effective to secure communication as the PN codes are pseudo random noise sequence codes.
In a mobile communication system based on the CDMA, a transmitting base station repeatedly transmits a PN code as a pilot signal for acquiring and maintaining synchronism in mobile stations and clock signal reproduction. A receiving mobile station detects pilot signals transmitted from a plurality of base stations, and assigns detected timings to respective demodulators. In the mobile station, a PN code is generated in the demodulators. The demodulators multiply the spread-spectrum signal transmitted from a desired base station by the generated PN code at the assigned timing to demodulate the spread-spectrum signal, respectively.
In the mobile communication system based on the CDMA, therefore, the base stations transmit PN codes at different timings as pilot signals. The mobile station detects the timing of the pilot signal supplied from the desired base station, synchronizes the PN code generated in the demodulators with the detected timing, and desperados the received signal using the PN code, for thereby properly demodulating only the spread-spectrum signal transmitted from the desired base station.
The mobile station receives radio waves under conditions that vary momentarily. Practically, the base station and the mobile station hardly communicate directly with each other without any obstacle interposed therebetween. The base station receives radio waves reflected by obstacles such as buildings and the ground, or radio waves reflected by remote mountains.
Therefore, the radio wave received by the mobile station is composed of a combination of plural reflected waves. Since the mobile station moves at varying speeds, the intensity of energy of a signal from the base station that is detected by the mobile station varies momentarily, occasionally resulting in a large attenuation of the received radio wave. Such an environment for radio wave reception is referred to as a fading environment.
The mobile station continuously detects the timings of pilot signals from base stations, and assigns the detected timings (referred to as multipaths) to a plurality of demodulators thereof for signal reception. In the fading environment, since the detected timings are frequently lost or regenerated, the demodulators are often unlocked and timings are often reassigned to the demodulators.
When a demodulator is unlocked, it is general practice to reassign a timing to the demodulator according to the following processing sequence:
(1) The demodulating process of the demodulator is temporarily stopped.
(2) A path (the timing of a PN code) that can be reassigned is selected based on the detected timing from a timing detector.
(3) If a path can be reassigned, then the timing at which the PN code is generated in the demodulator is synchronized with the detected timing.
(4) The demodulating process of the demodulator is permitted.
(5) The demodulator is locked.
According to the above processes, however, it takes a certain period of time before the demodulator is finally locked again for some reasons described below, and the quality of the signal received during such certain period of time is lowered.
(1) Since the demodulating process of the demodulator is temporarily stopped, the reception level of the demodulator is cleared once.
(2) To obtain the resulting detected timing from the timing detector needs to be awaited.
(3) It is time-consuming to select a path that can be reassigned.
(4) It is time-consuming to change into the designated timing in the demodulator.
(5) It takes a certain period of time until a locking threshold is achieved after the demodulating process of the demodulator is started.
Consequently, the mobile communication system based on the CDMA with each mobile station employing a plurality of demodulators has strongly been required to establish a process of receiving stable, high-quality signals.
It is therefore an object of the present invention to provide a method of stably receiving signals from base stations for producing high-quality received information even in a fading environment where multipaths of detected pilot signals are often lost and regenerated, and a radio communication terminal device for carrying out such a method therein.
It is known in the related art that a path which causes a demodulator to be unlocked is frequently lost and regenerated as it has multipath timing. Therefore, the regeneration of a path at the time the demodulator is unlocked can well be expected.
It has been found as a result of actual measurements that the period of time consumed for re-receiving and re-demodulating a signal which caused the demodulator to be unlocked is shorter than if taking the above processing sequence (1)-(5) to stop the process once for demodulation, though the period of time depends on the period of fading, i.e., the speed of travel of the mobile station terminal device.
In actual reception environments, since new paths are generated and lost momentarily, it is desirable to demodulate a newly detected path with stable strong energy rather than expecting the recovery of a regenerated path.
Therefore, if there is a multipath that can be assigned to a demodulator which has been unlocked, the demodulating process of the demodulator is temporarily stopped, and a new path is assigned to the demodulator, otherwise the recovery of a path which has caused the demodulator to be unlocked is awaited if there is no multipath that can be assigned to a demodulator which has been unlocked. In this manner, it is possible to receive high-quality signals stably.
A method of receiving a spread-spectrum signal and a radio communication terminal device which carries out the method according to the present invention detect when at least one of demodulators which demodulates a received spread-spectrum signal is incapable of properly demodulating the received spread-spectrum signal, and determine whether there is a demodulator which is not being used other than the at least one of the demodulators. If there is a demodulator which is not being used, a demodulating process of the at least one of the demodulators is continued.
If there are not enough paths that can be received and demodulated to be assigned to a plurality of demodulators, then the radio communication terminal device waits until the signal which has caused the demodulator to be unlocked is received again for being demodulated. In this manner, the time in which the demodulator is unlocked is minimized, and a stable, high-quality signal can be received.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.